Single-site polyolefins have narrow molecular weight distribution and uniform composition distribution (i.e., the comonomer recurring units are uniformly distributed along the polymer chains). The combination of narrow molecular weight distribution and uniform composition distribution distinguishes single-site polyolefins from conventional polyolefins made by Ziegler or chromium catalysts. Compared to Ziegler polyolefins, single-site polyolefins have improved impact resistance, tensile strength, and optical properties.
However, the uniformity of molecular weight distribution causes reduced thermal processability of single-site polyolefins. It is difficult to process single-site polyolefins under the conditions normally used for Ziegler polyolefins. The reduced processability limits the development of single-site polyolefins because altering process conditions often requires a large capital investment. Accordingly, it would be highly desirable to prepare polyolefins which possess the improved physical properties offered by single-site catalysts and also exhibit processability characteristics which are similar to those of conventional polyolefins.
One approach to achieve this objective is using mixed catalyst systems. For instance, U.S. Pat. No. 5,747,594 teaches a two-stage polymerization process. In a first stage, ethylene and a higher α-olefin are polymerized with a single-site catalyst. The polymerization continues in a second stage where a Ziegler catalyst is used. Therefore, the product is a mixture of single-site polyolefin and Ziegler polyolefin. The disparity of the two polymers in molecular weight and composition gives the product an improved thermal processability. Also, U.S. Pat. No. 6,127,484 teaches a multiple reaction zone process that uses a single-site catalyst in a first reaction zone and a Ziegler catalyst in a later reaction zone.
Another alternative is using a single-site catalyst in two different polymerization reactors which are operated with different activators. For instance, an alumoxane is used in one reactor and an ionic activator is used in the other. The use of different activators result in polyolefin made in the different reactors having different molecular weights and thus the combined polyolefin has a broad molecular weight distribution and improved processability. See U.S. Pat. No. 6,372,864.
However, the use of mixed catalysts or activators is generally associated with operability problems. The two different catalysts or activators may interfere with one another. For example, the organoaluminum compounds which are often used in Ziegler catalyst poison single-site catalysts. Therefore, catalyst deactivation is often involved when two incompatible catalyst systems are used. Catalyst deactivation is costly and complicated. See U.S. Pat. Nos. 5,371,053 and 5,442,019.
In sum, new catalyst systems are needed. Ideally, the catalyst system would produce polyolefins that have bi- or multi-modal molecular weight distribution. Ideally, the catalyst system would produce bi- or multi-modal polyolefin in a single stage or single reactor process.